A study of a lightwave circuit in which optical waveguides, Y branch optical waveguides, and directional couplers each composed of silica (glass) or polymer or the like are brought into integration on a substrate, has recently been brought into activation with a view toward aiming at reductions in the sizes and costs of optical parts. The Y branch optical waveguide is important as an element which constitutes the lightwave circuit. In order to reduce its branch excess loss, Y branch optical waveguides having various structures have been discussed.
While a symmetric optical waveguide whose branching ratio is 1:1, has been usually discussed as a Y branch optical waveguide, an asymmetric Y branch optical waveguide having a different branching ratio is required according to uses. An asymmetric Y branch using a silica (glass) waveguide has been described in, for example, "NTT R&D", Vol. 46, No.5, 1997, pp.473-485 (Article 1) or "Proceedings of the 1995 Electronics Society Conference of the Institute of Electronics, Information and Communication Engineers", SC-1-15, pp.337-338 (Article 2). An optical transmission and reception module excellent in received sensitivity and a splitter having an arbitrary number of branches have been implemented by using such an asymmetric Y branch optical waveguide.
Described specifically, the former (Article 1) example has an input waveguide, a tapered waveguide for extending or spreading incident light, and two output waveguides. Further, the branching ratio is rendered asymmetric by shifting the input waveguide and the tapered waveguide from a central axis by a predetermined value. In the latter (Article 2) example on the other hand, the width of a core at each input terminal or end of a branch optical waveguide is set as W1 and the widths of cores at two output terminals or ends are respectively set as W0. Further, a tapered waveguide is provided in which each core width gradually spreads from W1 to W0 along a light traveling direction L. The branching ratio is made asymmetric by setting an increase ratio dW/dL of each core width to different values with two waveguides which are caused to branch off.